Neodymium glass laser having room temperature output at wavelengths shorter than 1060 nm.

ABSTRACT

LASERABLE MATERIAL DOPED WITH A QUANTITY OF NEODYMIUM IONS IN A LOW CONCENTRATION WHICH RESULTS IN THE GLASS EXHIBITING A RATIO OF FLUORESCENT INTENSITY PEAKED AT 920 NANOMETERS OVER THE FLUORESCENT INTENSITY PEAKED AT APPROXIMATELY 1060 NANOMETERS OF AT LEAST .4 AS MEASURED BY A CARY MODEL 14 SPECTROPHOTOMETER. THE GLASSES ENABLE THE GENERATION OF LASER LIGHT IN A WAVEBAND WITH AN OPTICAL CENTER AT ABOUT 920 NANOMETERS AT ROOM TEMPERATURE (APPROXIMATELY 20*C.) WHEN POSITIONED IN A LASER CAVITY WHICH IS RESONANT AT 920 NANOMETERS. TWO SUCH LASERABLE GLASSES ARE GIVEN BELOW IN WEIGHT PERCENT:   BAO 26.6 59.0 AL2O3 8.8 7.8 GEO2 63.6 7.8 ND2O3 1.0 1.0

Aug. 20, 1974 W ETAL 3,830 747 NEODYMIUM GLASS LASER HAVING ROOMTEMPERATURE OUTPUT AT WAVELENGTHS SHORTER THAN 1060 NM. Original FiledMarch 10, 1971 FIG. 2.

65 4 3 W S o m T I E I M. O o a N 0 A ml N D W m g m M 9I| 5 G N 2 N I 29 E G .i L 6 2 5 E O 2 0 0 V I Q 9 A I 9 W 2 2 .4 u 3 2 l. O F .l. O O OO 4. 4 rtmzw z m k 4mm 4 I. 3 m 3 G ulv P F MI U P. I' I' 0 4 2 3 G .WHUH O n m w m 9 n. a 0 9 m o O 7 w n m a United States PatentO US. Cl.252-3014 F 2 Claims ABSTRACT OF THE DISCLOSURE Laserable material dopedwith a quantity of neodymi- 11m ions ina low concentration which resultsin the glass exhibiting a ratio of fluorescent intensity peaked at 920nanometers over the fluorescent intensity peaked at approximately 1060nanometers of at least .4 as measured by a' Cary Model 14spectrophotometer. The glasses enable the generation of laser light in awaveband with an optical center at about 920 nanometers at roomtemperature (approximately 20 C.) when positioned in a laser cavitywhich is resonant at 920 nanometers. Two such laserable glasses aregiven below in weight percent:

ONmQ

This application is related to Application Ser. No.

' 122,724, filed Mar. 10, 1971, now U.S. Pat. 3,711,787

entitled Neodymium Glass Laser Having Room Temperature Output atWavelengths Shorter than 1060 Nm. by 'E. Snitzer, C. Robinson and R.Woodcock. The subject matter thereof is incorporated herein byreference.

BACKGROUND OF THE INVENTION This is a division of application Ser. No.122,723, filed Mar. 10, '197l,'now US. Pat. 3,714,059.

For many applications it is considered desirable to have a laser devicecapable of producing an output of laser light energy at wavelengths of920 nanometers. The desirability of generating light at this wavelengthis notable with systems utilizing detectors since there are detectorsavailable which are extremely sensitive at this wavelength. Crystalsexhibiting such emission are known. For examplefa YAG crystal laser isdescribed in an article entitled Oscillation and Doubling of the 0946 1.'Line in Nd zYAG which appeared in Applied Physics Letter, Vol. 15, No.4, Aug. 15, 1969, page 111. A problem, however, with the YAG laser isthat it is a crystal -'and thus does not possess the numerous advantagesthat are known to' be attendant with glass lasers.

Gla'ss has 'various characteristics which can make it an ideal laserhost material. It can be made in large pieces of diffraction-limitedoptical quality, e.g., with an index refraction variation of less thanone part per million across a 2.5 cm. diameter. In addition, glasslasers have been made in a variety of shapes and sizes from-fibers a fewmicrons wide supporting only a single dielectric waveguide mode, to rods2 meters long or 7.5 cm. in diameter. Furthermore, pieces of glass withquite difierent optical properties can be fused to solve certain3,830,747. Patented Aug. 20, 1974 There are two important differencesbetween glass and crystal lasers. First, the thermal conductivity ofglass is considerably lower than that of most crystal hosts. The secondimportant difference between the glass and crystal lasers is theinherently broader absorption and emission lines of ions in glass. Thesebroader lines imply greater pump-1ight absorption, greater energystorage and much reduced spontaneous self-depletion for a given energystorage.

A glass laser has been suggested which exhibits an output at about 9180A. from neodymium active ions. Such a laser device is described in US.Pat. No. 3,270,290 by R. D. Maurer. However, in the Maurer patent thedevice is not taught to be capable of laser emission at roomtemperature. In connection with room temperature operation, it is wellknown that it is desirable to utilize a laser at room temperature inorder to eliminate cumbersome equipment otherwise necessary to cool thelaser device.

SUMMARY OF THE INVENTION In accordance with the present invention aneodymium doped laser glass is provided which enables generation oflaser radiation at about 920 nanometers at room temperature.

Accordingly, it is an object of the present invention to provide newneodymium doped laser glass devices which are capable of operating atroom temperature and which will generate laser light energy in wavebandwith an optical center at about 920 nanometers.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an emission curve showing thefluorescent emission properties of glasses utilized in laser devices ofthe present invention;

FIG. 2 is a schematic representation of the various energy levels in aNd+ ion;

FIG. 3 is a diagrammatic illustration of a laser device of the presentinvention;

FIG. 4 is a transmittance and reflectance curve of a reflector useful inthe laser device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the presentinvention, a laser device is provided which is comprised of a neodymiumdoped glass laser host positioned within an optically regenerative lasercavity. It has been found that trivalent neodymium ions in glass hoststypically have emission curves of the general shape shown in FIG 1. Thiscurve is provided at the outset to illustrate properties which areuseful in carrying out the object of the present invention.

The flourescent curves shown were measured in a Cary 14spectrophotometer by placing the glass sample in a copper fixture whichin turn was placed in the sample compartment of the Cary. The glass wasirradiated at right angles with a xenon arc lamp through a filter whichblocked the transmission of wavelengths longer than approximately 800nm. Thefluorescent spectrum was recorded using the automatic slitcontrol which adjusted the slit width so that the output of a coiledtungsten filament lamp with a filament temperatureof approximately 2800K. produced a constant deflection on the recording chart for allwavelengths. Thus the recording chart must be corrected to obtain thetrue relative intensities by dividing the chart deflection bya factorproportional to the energy radiated by the tungsten lamp at thewavelengths of interest. We have estimated the correction factor forobtaining the ratio of the 920 nm. fluorescent intensity to the 1060 nm.intensity to be approximately unity. This estimate was made byhusing e'tun'gsten emissivities measured by J C.- De -Vos (J. C. De-vos Physics20,690-(19 54 for a ribbon filament tungsten lamp operating at 2800 K.in a calculation of the energy radiated by the coiled filament lamp atthe:two-wavelengths of interest. Theintensity ratios reported here weremeasured directlyfrom the Cary charts using no correction factor.

In FIG. 1 a curve is shown with peaks 12 and 14 at 920 nanometersand-.1060 nanometers respectively. In connection with these andsimilar-peaks, it is. tobe understood that the-actual range ofuseful-fluorescent emission is somewhat. broad. In fact, in; accordancewith the invention peak 12 can have a band width of nanometers locatedbetween 905-925 nanometers, as is represented by spectral region A ofFIG. 1, while peak 14 can have a width of 20 nanometers located between1050-1070 nanometers, as is represented by spectral region B of curve10. Although curve 10 shows other peaks, for purposes of the presentinvention the entire peaks represented by spectral regions A and B ofcurve 10 are the most critical. Numerous tests have indicated that whena neodymium doped glass host is positioned in a cavity with reflectorsthat suppress emission at 1060 nanometers, peaks A and B are the onlypeaks that need be considered in evaluating whether the laser will emitat 1060 nm. or 920 nm. at room temperature. Although a ratio of A/B ofat least .4 produces operative results, that is, laser action at 920nanometers at room temperature (20 C.), it is to be understood that inaccordance with the invention the greater the magnitude of the foregoingratio the more effective will be the host for producing the desiredlaser emission when positioned in the cavity of the present invention.

As indicated above, in addition to considering the emission spectra ofthe host glass, consideration must also be given to the opticallyregenerative laser cavity into which the host glass is positioned. Inaccordance with the invention the reflectors forming the laser cavitymust suppress laser emission at 1060 nanometers. It is to be understoodthat such reflectors are available and that the reflectors per se formno part of the present invention. For example, dichroic reflectors areavailable which transmit approximately 85% of the light at 1060nanometers while reflecting approximately 99.7% of the light between therange of 800-1000 nanometers.

Although not intended to be restricted to a particular theory, anunderstanding of the energy level scheme including the 4 manifold of theNd ion is useful in explaining the present invention. In this regard,FIG. 2

is provided as a schematic representation of the various energy levelsin a Nd ion.

A condition necessary for laser action according to this invention isthat the population of the initial state be at least as large as theterminal state which requires, therefore, that the initial statepopulation be at least 0.033 of the total population in the groundmanifold 19,.)-

The cavity losses for the 1060 nanometers emission must be higher thanthose for the 920 nanometers emispresent invention, two extremeexperiments were conducted. In the first experiment a reflector R (32 ofFIG. 3) which was 98% reflective for light at-920-nanometers and 98.4%reflective at'1060' nanometers, was employed as one"r'eflector with 'asecond reflector R (34 of FIG. 3),which 'was 99.5% reflective at 920nanometers and 15% reflective at 1060 nanometers. With'the combinationof reflectors R and R laser action at 920 Column 3:

: nanometers was observed with the host glass o Example 1 6. In a secondtest-with. both reflectors-32- and-34 being R types, laser emission at920 nanometers occurred even more readily.

The foregoing tests, as well as other tests, proved that when the ratioof A/B= discussed above is greater than .4, the laser device willgenerate laser fli'glit'-energy"at abo'ttt"920iriano1iieters-.atrboniltempef weav -o.) if the device includes reflect jrnp atibl thatwavelength and which 'suppressfl'a'ser, emission'at- 1060 handmeters. Apump' light sofirce"is ndt shown F it being understood that many, pumpsources al'e 19, 6 which will produce theflre'quired" populationinversionin the neodymium ion. One such pump so urjce commonly employedis a xenon flash tube. In" this regard, the ha rdware for producingenergy inversions;-are conventinal and form no part ofrthepresentinvention.

The transmittance and reflectance. curve oflthe R type reflectors isshown in FIG. 4 of the drawin'gl'Such a refiector is available fromSpectra-Physics, 12'50 -West Middlefield Road, Mountain View,"Calif.94040. I

The laser glass as set forth in*Example "16, as well 'as the otherexamples, a're'preferably formed in the following manner. Thealkaliearth and'alkaline earth metals are added to the batches nitratesor carbonates and all other constituents of the finished "glass (silica,neodymium, zinc, boron, antimony) areadded directly as oxides. Theconstituents are added in the known'stoichiometric amounts to yield aglass having a final composition asset forth in the various examples."The glass making raw materials must be of high purity and, inparticular, must be free of contamination from iron or other elementswhich would cause light absorption at the desired laser emissionwavelength if they were present in the finished glass. The finishedglass for example, should not contain more than 5 parts per million'ofiron as Fe O The glass may be prepared by fusing the' raw materials inaceramic crucible heated in a "Globar electric-'fur-nace.-No specialatmosphere is necessary in} the furnace-Theraw materials are mixedintimatelyand' as completelyas possible in a mixing device that does notintroduceany contamination. The mixed batch is loaded into a high"purity ceramic crucible which will not contaminate the melt withundesirable impuritiesf'The crucible should be at a temperature ofapproximately 2700 F. when the raw .material is charged, theloadingoperatipn taki'ng' iapproximately two hours since the leveli nthe crucible drops "as the batch materials fuse together to form theglass and thus require the addition of more batch. When thec'ilti'r'ging of the batch is compl'eted, the temperature of the melt israised to approximately 2800 F. andisfheld at 'this temperature for onehour'to free the melt of stria'e. The temperature of the glass is thenlowered approximately 2700 P. where it is maintainedifor period of aboutone hour before casting The teinperaturevalue last recitedis suitablefor a melt of 1 1b. butlit is to beunde r stood tliat the preferredtempera-ture 'at casting isa functionoifi the size of the cast with'larger casts" requiring lower temperatures for control of the glass.The'glass may be cast in a cast iron mold, and'is transferred anannealing, oven just as soon as it has cooled enough to maintain itsshape. The glass is annealed at a temperature'of 1100" for one hour andis then cooleddown slowly overnight 'to room temperature. i

' In the following examples:

Column 1=componentsin finished glass I Column 2=percent by weight ofcomponents in'the finished glass... 7 I

fluorescence intensity peaked ,betwee'n' 905 nanometersto 925 nanometersp fluorescence intensity peaked. between l050 nanom etersto 107,0nanometers (referred to in the specification as A713 Column 4=figure ofthe drawing showing relative fluojrescence' intensity curve EXAMPLE 10In accordance with the invention it has been discovered that theneodymium ion concentration is the most important factor to consider inorder to obtain a laser glass with an A/B ratio greater than .4. Fromthe foregoing examples and numerous tests, .1-3 wt. percent of Nd O inthe final glass results in a laser glass which is usable in accordancewith the invention. The best results, however, result from a glasscontaining .5-1.5 wt. percent Nd203.

In accordance with the invention, operative results occur when theneodymium ion concentration is kept low, as is the case with theforegoing examples. It has also been discovered that with Nd-silicateglass lasers, heavy monovalent alkali ions, that is, potassium, rubidiumand cesium, when included in the glass to replace lighter alkali ions,that is, sodium and lithium, improve the overall results. That is. theratios of fluorescent intensity peaked at 920 nanometers over thefluorescent intensity peaked .at 1060 nanometers (A/B) greatly in'excessof .4 are possible. Thus, A/B ratios of silicate glasses containingneodymium in the range of .1 to 3 wt. percent as laser active ingredientare substantially increased by the use art Nd doped laser glasses,However, heavier divalent ions such as lead, cadmium and' strontium havebeen found to lower the foregoing ratio to a lesser extent when"compared to the elfect caused by light divalent ion suchas calcium.Barium has been especially desirable in "increasing the foregoing ratioin silicate glasses with trivale'ntneodymium within the range of .l-3'wt. percent. In this regard, usable glasses may include bariumo ide-inthe range of -10 wt. percent with approximately wt; percent" beingpreferred. A range of. 0=10 wt.l.percent islsuitableionoth'er divalentions usable in the glass-composition.

Tests show that ini-isilicate glass compositions contain- Bese Si1icate.v w

Balance Divalent ion Monovalent ion.. Laser ion.

ah'ge in percent y wt. finished glass Constituent ing more thanapproximately weight percent of alkali, a given molar percentage of Cs Ois superior than the Divalention same molar percentage of Rb O which inturn is better MOHOYBlBHt i011 Rubidium oxlde- 7- 0 h th 1 f fLaser1on-...-. NchO- .1-3 t an e same mo ar percentage 0 K 0 mso ar asan Base $818M, increase of A/B 1s concerned. From the standpoint of 1maximizing the A/B ratio of a laser component, it would 1 be desirableto have all of the alkali provided by use of TABLE 9 cesium or rubidiumin relatively large weight percentages i i of the order of 20%. if;3355* Composition ranges for Nd-silicate base glasses which Constitumfinished c1888 promote high A/B ratios are given in the tables below:Divalentqm ar m Mme Monovalent ion Cesium oxide-- 7-20 Laser ion-..Ndz0a TABLE 1 Base Silicate....

Range in percent 7 C it by .in v v onst uent finished class KBLEDivalent ion Lead oxide 0-10 Monovalent ion Potassium oxide 7-20 Laserion- Nd20a- 1-3 Base" 7 Sflimm Balance Divalent ion -.i.-.. lidonogalention.. N L aser on- TAB E 2 Base Silicate.

Range in percent by wt. in Constituent finished glass Divalent ion. Leadoxide 0-10 Monovalent ion Rubidium oxide- 7-20 Laser ion NdzO; .1-3 BaseSilicate- Balance 40 v Constituent Divalent ion Strontium orida...' ITABLE 3 Monovalent ion Rubidium oxide- Laser ion.-. Ndq0 Range inpercent Base Silicate Balance bywt.in Constituent finished class I l:Iivalant iogiiflu gead oxidea. H r a l OHOVB CH 011 65111111 OX! 6Laser ion- Ndi .1-3 TABLE 12 Base- Silicate Balance g t g v it, yConstituent Y W finished glass TABLE 4 Divalent ioni Strontium oxid --L0-10 g a; g fg 23??? $532 F153 Constituent finished glass f'ff 7 IBalance Divalent ion Cadmium oxide...- 0-10 Monovalent ion Potassiumoxide- 7-20 Laser ion NdzO; 1-3 Base- Silicate- Balance V TABLE 1 Raiigegit piercent a; Y n 5. I Q Cons tituent' iinishedclass l e gga- 3559Divalent imr r mi mriria 0-10 v I v Monovalention..- 'xture of two'o'rmore of pota's- 7-20 Constituent i t -finished class L Ng g and mm (1mm.1 3 Divalent ion...- H 3 i Monovalent ion d5 v .,h,. ...n.

lTAi% ;E-iii1.- TABLE 6 I i. l, .7 w r Range in percent Range in percentby wt. in by wt.in. Constituent nished glass 7 Constituent finishedglass Divalent ion...'-.- Cadmium oxide 0-10 Divalent ion Cadmiumoxide..- (3-10: Monovalent ion..- Mixture of two ormore oi pota 7-20Monovalent ion Cesium oxide. 7-20 siumyeesium and rubidium. -Laserirm N01 .1-3 Laser nn "Ndz .1-3

Base Silk-rain J:

TABLE 15 Range in percent by wt. in

slum, cesium and rubidium Laser ion Base It should be understood thatthe term silicate base as used throughout this specification and claimsis generic to pure silica, SiO and other known silicate bases, such asalumino-silicate bases. In this regard, various base modifiers arefining agents are contemplated to be included in the silicate bases ofthe foregoing examples.

From six different glass compositions chosen from the foregoing tables,laser rods 2 /2" long x 4 mm. diameter were fabricated. For this rangeof concentrations the threshold for laser action varied from 100 to 137joules input respectively.

Verification that these rods were lasing at 0.92 m. and not 1.06 ,am.was made using an infrared image converter and appropriate narrow bandpass filters for the Wavelengths in question.

When the laser rod is to be utilized in a laser system wheresolarization is a problem, the rod containing the low concentration ofNd O may be appropriately clad to prevent the solarization of the laserrod. It is to be understood, however, that a clad rod is not part of thepresent invention.

The invention may be embodied in other specific forms Without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

We claim:

1. An inorganic laser glass exhibiting a ratio of fluorescent intensitypeaked at approximately 920 nanometers over fluorescent intensity peakedat approximately 1060 nanometers of at least 0.4, the glass consistingessentially of the following constituents in percent by weight:

B30 26.6 A1203 8.8 Ge0 63.6 Nd O 1.0

2. An inorganic laser glass exhibiting a ratio of fluorescent intensitypeaked at approximately 920 nanometers over fluorescent intensity peakedat approximately 1060 nanometers of at least 0.4, the glass consistingessentially of the following constituents in percent by weight:

BaO 59.0 A1203 7.8 (360 32.2 Nd203 1.0

References Cited UNITED STATES PATENTS 3,320,043 5/1967 MacKenzie 106-473,270,390 8/1966 Maurer 252301.4 F 3,422,025 1/1969 Snitzer et a1.252301.6 F 3,624,547 11/1971 Dugger 252301.4 F 3,674,455 7/1972 Dugger.

OTHER REFERENCES Glass Industry, September HELEN M. MCCARTHY, PrimaryExaminer US. Cl. X.R.

10647 Q; 252301.4 R; 33194.5 E

